In recent years, coil tubing has been used in downhole operations with greater and greater frequency and for a greater variety of jobs. Also, coil tubing has found application for operations at ever increasing depths. All of these factors have led to the use of large sizes of coil tubing. Coil tubing of greater diameter and wall thickness, as well as the greater length, requires sturdier and more robust BOPs at the well head when used with coil tubing.
The use of BOPs in coil tubing operations in the oil and gas field is well know. Such blowout preventers generally include a housing with a bore extending through the housing. Opposed chambers extend laterally of the bore in the housing and communicate with the bore. Rams are positioned in the chambers and the rams are connected to rods that are supported for moving the rams inwardly into the bore to close off the bore. This action divides the bore into a zone above the rams and a zone below the rams. The rods also serve to retract outwardly from the bore to open the bore.
Various types of rams may be employed such as those which engage circumferentially around the coil tubing for sealing engagement with the tubing, while others are provided with cutting surfaces for shearing the tubing which extends through the bore of the blowout preventer.
Among other uses, BOPs are commonly used in coiled tubing systems as a means of holding the tubing and isolating the well bore pressure during a variety of conditions, including emergencies. The configuration of the BOP rams and side port facility allow well-control operations to conducted under a variety of conditions.
Newer blowout preventers include four sets of rams, which may be referred to herein as a “Quad BOP”. The system comprises a set of four stacked elements, each with a different function. Blind rams are shut when there is no tubing extending through the body of the BOP. Shear rams are designed to close on and cut through the tubing. Slip rams close on and hold the tubing, ideally without damaging the surface of the tubing member. Finally, pipe rams seal around the tubing when it is in place.
BOPs used at the wellhead for heavy walled drill pipe, for example, commonly use complex pressure regulations systems to control hydraulic control pressure to each of the rams in the BOP. However, such is not the case with BOPs used with coil tubing. In the art today, a single source of hydraulic actuation fluid at a single pressure is typically applied to all of the rams in the BOP. Further, the ram requiting the greatest pressure is the shear ram, and thus shear ram pressure is typically applied to all the rams in the BOP. With larger and heavier coil tubing, using higher shear ram pressure, this fact presents a problem when a lighter gage coil tubing is run into the hole. Applying the higher hydraulic actuation pressure to a slip ram to simply hold the tubing in place has been found to score or otherwise damage the coil tubing.
Although slip rams ideally do not damage the tubing surface of the tubular member through th BOP, it has been found that even a single actuation of the slips against the tubing can score the exterior surface of the tubing. In today's high performance operations at elevated pressures, this scoring can reduce the useful lifetime of the coil tubing. Thus, there remains a need for a means for applying a lower actuation pressure to a slip ram, while retaining the availability of the higher hydraulic pressure for the shear rams. Such a means should be simple, reliable, adjustable, and robust.